There are many uses of for head-mounted visual display devices. In the past most of these uses have been military where a typical use is a helmet-mounted display. Many of these military devices display an image for only one eye and superimpose the display over a live background so that the user looks "through" the display. Other "dual image" devices simultaneously display two images, one for each eye. Such conventional military devices are mounted on the helmet which is commonly worn in military applications and allow the user to fly aircraft or operate equipment while receiving information from the display.
Recently, other applications for dual image displays have arisen. These newer applications include "virtual reality" displays in which the displays are not transparent--instead, the images generated by the displays are substituted for the live image. Such displays may be combined with head position sensors so that the images presented to the user change as the user moves his head. Virtual reality displays allow a computer which controls the displays to effectively control a user's visual environment.
Both conventional military displays and prior art virtual reality displays have used one of several types of prior art technologies. One type of "conventional" helmet-mounted dual-image display uses a pair of conventional cathode ray tubes (CRTs) to generate the two images. Generally, the CRTs are mounted on the user's helmet, and an optical system allows the wearer to see a magnified image of the CRT image by projecting the image onto the inside surface of the helmet visor. In order to attain reasonable resolution, it has been necessary to use miniaturized high-resolution CRT tubes which have been heretofore so expensive that their use has been limited to military applications. In addition, such displays are often bulky and heavy, causing neck fatigue, and place high voltages and powerful magnetic fields close to the user's head.
Other conventional head-mounted displays use liquid crystal displays (LCDs) passive matrix LCD panels or other flat panels. These latter displays are conceptually similar to the CRT based devices, but use one or two flat panel displays instead of CRTs to generate the visual images. These flat panel displays have an advantage over CRT displays in that they are generally lightweight and inexpensive making them suitable for uses such as consumer-oriented virtual reality displays. However, they suffer from additional drawbacks. In general, presently available flat panel displays generate poor quality images when compared to CRT displays. In addition, passive matrix LCD panels suffer from long image persistence, so that quickly-moving images are perceived as badly blurred.
Another conventional type of dual-image head-mounted display uses CRTs or LCD panels which are mounted near the user, but on a fixed platform, to generate the images and a coherent fiber-optic bundle to carry the image to the helmet for display. In addition to the drawbacks discussed above, the high cost of coherent fiber bundles has limited the use of this latter type of display especially in relatively low-cost consumer applications.
Another problem which has arisen in the use of the aforementioned conventional dual-image displays in virtual reality devices is that the user commonly perceives a clear demarcation between the dark background of the image area and the surrounding dark non-image area. It has been found that the experience of "virtual reality" is enhanced if this demarcation is not perceived or is minimized.
Another type of miniaturized display which is suitable for dual-image displays and especially for low-cost consumer virtual reality displays is a mechanical scanning display. This latter display uses a line of light-emitting devices to generate one line of an image. The line image is reflected from a mirror to a user's eye. By mechanically moving the mirror as the line image changes, a raster scan image can be generated. Suitable optics allow the display to be miniaturized to the point where it can be conveniently mounted on the user's head. This type of display is described in detail in U.S. Pat. Nos. 4,934,773 and 5,003,300 assigned to the assignee of the present invention. The disclosure of These latter patents is incorporated herein by reference.
These latter displays have high contrast and are thus particularly suited for virtual reality devices because the demarcation between the image and the background is minimized. However, since the displays utilize mechanically moving mirrors, mechanical vibration is a problem. In displays which generate only one image, the mechanical vibrations can be minimized by careful design of the mirror supports, the use of a counterbalance and symmetric drive motor. Such an approach is described in detail in U.S. Pat. No. 4,902,083 assigned to the assignee of the present invention. However, when two separate displays are used to generate two images, for example, in virtual reality systems, the vibration problems are increased because the two displays may interact to generate a low-frequency "beat" vibration which is particularly annoying to the user.
Accordingly, this invention provides a light-weight dual image display mechanism that can be worn like a pair of glasses and can be configured as both a transparent and an opaque display. The display is suitable for use as a virtual reality display because it has very high contrast, so that there is no clear demarcation between black background of image area and the surrounding background and the display image has a low persistence to allow head tracking without image smear. The scanning mechanism is designed to reduce vibration in dual image head-mounted applications so that a dual image display can be combined with a head motion tracking mechanism to cause the display to respond to head motion. The dual image display architecture is designed to efficiently share electronics between both displays. In addition, the mechanism can be designed to display images in full color.